The present invention relates to a liquid crystal display device, specifically to an active-matrix liquid crystal display device which provides a specific connection structure for a plurality of thin film transistors for driving pixels, the transistors being connected to one of the transparent electrodes which forms the pixel.
In a conventional active-matrix liquid crystal display device, as shown in FIG. 4, one thin film transistor b, for each driving pixel, is connected to each of the transparent electrodes (Indium Tin Oxide) arranged in a matrix fashion to form each pixel. A drain electrode c, of each thin film transistor b, is connected to each transparent electrode a, and each source electrode d is connected to a common source electrode line e on each column of the matrix. Further each gate electrode f is connected to a common gate electrode line g on each row of the matrix, respectively. The above-described transparent electrodes a, thin film transistors b, source electrode lines e on each column, and gate electrode lines g on each row are formed on an insulating substrate h.
In the liquid crystal display device having the above-mentioned structure, the characteristic inspection of the thin film transistor b is conducted by bringing a needle-shaped probe into contact (pressure contact) with the transparent electrode a of the pixel, which is connected to the drain electrode c of the thin film transistor b. Then, a voltage is applied to the source electrode d, which is greater than or equal to a threshold voltage applied to the gate electrode f, and the resulting current flow into the transparent electrode a is measured.
However, with the inspection method described above, it is difficult to test all of the thin film transistors because of the presence of narrow pitches of the transparent electrodes a and because of the large number of pixels. In addition, the contact of the needle-shaped probe and the transparent electrode of the pixel is prone to damage the thin film transistor b, and in the case of a color liquid crystal display device, in inspecting the characteristics of the thin film transistor b, a defect in the pixel would have been judged to have occured by performing black-and-white display, and therefore the manufacturing cost would be high.
Also, in the Japanese Patent Laid-Open Publication No. 61 (1986)-249078, a matrix-type display device is described wherein two thin film transistors are disposed for one pixel electrode, and a bypass is disposed in the source electrode line, and either of the thin film transistors is utilized in a method of correcting a defect caused by a short-circuit between the gate electrode line and the source electrode line, a short-circuit between the source electrode and the drain electrode, a short-circuit between the gate electrode and the drain electrode, or a breaking between the drain electrode and the pixel electrode. Therefore, the reliability of the display device can be secured.
However, in the matrix-type display device as shown in the above-mentioned patent publication, when a defective thin film transistor is separated from the pixel electrode by etching or utilizing a laser cutter, this results in greater time lost and higher costs required for inspection and correction, especially in mass-production.
The present invention has been created in light of the above-mentioned circumstances, and has, as its purpose providing a liquid crystal display device which will allow characteristic inspection of the thin film transistor for a driving pixel without damaging the thin film transistor.